This invention relates to a brake valve including two control spools each providing for connection of a brake circuit to either a fluid source or an unpressurized reservoir, wherein the first control spool is displaceable mechanically by means of a brake pedal and the second control spool is displaceable hydraulically by the second control spool being assigned a pressure chamber which is in permanent communication with the brake circuit associated with the first control spool and into which the second control spool extends with an effective front surface. Such a brake valve is described in French Pat. No. 1,293,001.
In a brake valve so constructed, the brake pressure metered into the first brake circuit controls the control spool of the second brake circuit. In the event of failure of the first brake circuit because of a defect, it is possible to bring the first control spool into abutment with the second control spool by means of the brake pedal and to thus displace the second control spool mechanically so that actuation of the brake circuit associated with the second control spool continues to be ensured. When viewed from the safety angle, the brake valve of this type is required to actuate two brake circuits independently of one another.
A disadvantage of this prior art brake valve is that pressure is allowed to develop in the brake circuit associated with the second control spool only when the pressure in the first brake circuit has reached a magnitude sufficient to overcome the frictional forces of the second control spool. The result is that on brake actuation the second brake circuit always responds later than the first brake circuit. In practice, the second control spool's frictional forces cause pressure differences of 7 to 8 bars between the two brake circuits which is hardly acceptable because of reasons of safety. Particularly in brake systems with diagonally split brake circuits it is essential for the brakes of both brake circuits to respond concurrently and to have identical pressures in both brake circuits.